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Anthropology 130



Anthropology 130 (Online)

Human Variation and Adaptation
By Bonnie Yoshida

What does it mean to take an evolutionary perspective on human variation? It means that anthropologists are interested in viewing human traits through the lens of adaptation.  Adaptation is the successful interaction of a population with the environment.

Humans have different levels of adaptation to the environment depending upon the circumstances. This diagram is a good way of depicting the different layers of insulation from the environment. You can think of the outside environment as conditions that cause stress (red arrows). These can be abiotic or non-living forces such as climate or altitude. There are also biotic or biological stressors. These can be disease or a crop failure which results in food shortage. Or even social stresses like a state of war or the psychological stresses of living in modern society. All these things put stress on the individual, causing discomfort, illness or death.

model of human adaptation
Hierarchy of human responses to stress

Cultural Responses
man wearing fursHumans alone depend on culture and technology to respond to environmental stress. Cultural responses can be instituted immediately and are learned behavior (If youíre cold you can put on clothes, build fire, go inside, have some soup). Remember the difference between modern humans and Neandertals in their response to the harsh ice age environment? Neandertals adapted more with their bodies, but  modern humans used culture and technology such as bone needles to tailor clothing.

Immediate physiological responses (Acclimation)


Immediate physiological responses (shivering, sweating, etc). These are things your body does immediately to respond to a stressor. Goose bumps for example is a way for your body to conserve heat by constricting the blood vessels. Once the stress is eased, these processes are quickly reversible.

female bodybuilderShort term acclimatization
These are still short term physiological responses, that are reversible, but they donít take effect immediately (perhaps taking days to months to institute and reverse).  These processes are known as acclimatization. Tanning and bodybuilding are some examples of these.

If you stress your muscles to the limit by lifting weights, you will be able to increase your weight load and build muscle mass.  However, this increased muscle mass will not last--if the weight training ceases, your body will return to its original state.

Developmental plasticity

Developmental responses occur during the period of childhood growth. Sometimes this is known as man exiting cardevelopmental or phenotypic plasticity.  The capacity of humans to respond to environmental change is

How not only genetics but also our environment determines our phenotypic characteristics. A poor diet, or poor health care or a polluted environment can affect your biology. For example, take the big jump in height during the past hundred years or so. If any of you are the children of immigrants from non-industrialized countries, you are probably taller than your parents. This is probably due to better nutrition and health care in industrialized countries.

Which country has the tallest people in the world today? Read the article The Height Gap (from the New Yorker) to find out.

Genetic Responses

sickle cell anemiaAdaptations at the genetic level are changes that happen very slowly over many generations through natural selection. These can also include changes in a single allele.  The sickle cell allele is a classic case of a genetic adaptation related to two different disease processes.

You can read about the Sickle Cell case study on pp. 79-83).  It is the focus of the virtual lab associated with Chap. 4.

Also, check out this web tutorial on the evolution of sickle cell anemia at the PBS web site.

Genetic responses
can also include more complex variation such as differences in body form, physiology or skin color that involve multiple alleles and probably a significant degree of environmental conditioning as well.

A Case Study: Adaptation to High Elevation

High altitude adaptation is a good example of the hierarchy of responses used by humans to adapt to their environments. There are anthropologists specializing in human biology who study how some humans have adapted to living at extreme environments.

Tenzing Norgayquechua woman

Tenzing Norgay, a Sherpa Quechua woman of the Andes region

For example, they may ask why some human populations such as the Sherpas and mountain people of the Andes above display greater physiological adaptation to high elevations. The Sherpas of Nepal are an example of people who have adapted to living at extremely high altitudes where each lungful of air has only about a third as many oxygen molecules as at sea level. But how did they come to attain these capabilities? Do they carry genes that have evolved to help them survive better at high altitude conditions? Or does growing up in the environment program the body to adapt differently than a lowlander would?

When people who live at low elevation travel to high altitudes, they usually experience immediate stress on the body.  Most symptoms of high altitude stress are caused by hypoxia, which is lack of oxygen.  Read here to learn more about oxygen transport in the body. This results in symptoms like headache, nausea, insommia, loss of appetite, etc.  These would be immediate physiological responses to stress as described above. Have any of you ever experienced this upon ascent to a high elevation environment?

Eventually, after several days at altitude, you may acclimate somewhat. This is most likely due to an increase in your red blood cell count and changes in respiratory patterns and cell function. However, it is unlikely that you will attain the equivalent physiological function of someone who grew up at high elevation. If youíre born in the highlands, you have a somewhat deeper level of acclimatization at the developmental level. You tend to have greater lung capacity and be more efficient at transporting oxygen from blood to body tissues. So they donít need to form as many red blood cells. Developmental plasticity can also result in a body form better adapted to high altitude such as increased lung volume.

What about groups like the Sherpas or highland people in the South American Andes who have lived at extreme elevations for thousands of years? Have they evolved still deeper adaptations at the genetic level to high altitude?

Check out this National Geographic web article about current research on  human adaptation to high elevation.

Variation in Body Form

Now we will examine two areas where human phenotypic variation seems quite pronounced--variation in body form and in skin color.  As you'll see, both of these can be explained to some extent by adaptation to particular environmental circumstances.

Anthropologists have noticed that there seems to be a relationship between climate and body size in humans as well as other animals. Theyíve looked at human body size and shape applying two rules about animal body form that were developed by biologists--Bergmannís rule and Allenís rule.

Bergmannís rule.

Populations in cold climates tend to have larger body size than populations in warm climates. In other words, species who occupy a wide range of environments tend to have larger bodies in cold areas. Polar bears are larger in body size than brown bears who live in warmer climates. Why? Because a larger body loses heat less rapidly than a smaller body size. Why is that? Because a larger object has a smaller surface area to volume ratio. So as body size increases, volume increases at a faster rate than surface area (as you can see with the cubes below).

bergmann's rule
Illustration of Bergmann's rule

Allenís Rule

Allenís Rule refers specifically to the limbs or appendages of mammals.
Mammals in cold climates tend to have short, bulky limbs allowing for less loss of body heat. Mammals in hot climates have long, slender limbs allowing heat to dissipate easier. This is the same geometric principleólonger appendages increase surface area (see below).

allen's rule
Illustration of Allen's rule

So, do these rules hold true with modern humans? It seems to, to some extent.
Populations that inhabit hot climates tend to be very linear in build, and human populations in cold climates tend to be heavier and stockier.
inuit manEast Africans
You can see Allenís rule in practice with Eskimos who have a somewhat stockier body build and shorter limbs, as compared with East Africans like the Masai who have a long and linear body build.

But this doesnít hold true for all people. There are many exceptions to this rule. Thereís also a lot of variation within populations, and a lot of this is dependent on cultural variables such as diet. And developmental plasticity could also play a role in shaping body form (see above discussion of human height). So thereís probably a mixture of cultural, physiological and genetic factors at work here.

Variation in Skin Color

map showing skin color variation

Skin color variation is another example of adaptation and natural selection in human populations. You can see that there skin color varies with distance from the equator. What is responsible for skin color is the production of melanin, so variation in skin color is due to differences in melanin production. The amount of pigmentation in skin is greatest in the tropics, and lighter in northern areas.

So what explains this variation in skin color? There are several prevailing theories.
skin cancer
One with which you are probably familiar with is that dark skin helps protect against UV radiation that can lead to sunburn and skin cancer (above). However, one problem with this hypothesis is that most skin cancers occur relatively late in life, after a personís reproductive years are finished. In that case, the effect of natural selection would be limited.

According to another hypothesis, dark skin evolved to protect against another harmful effect of UV radiationóit destroys stores of folate (a member of the B-vitamin complex) in the body. Folate is necessary for normal cell division. Folate deficiency can have direct effects on fertility: namely, lowered sperm production in men and neural tube defects in the developing embryo during pregnancy.

If dark skin has such benefits, why wouldnít everyone simply have dark skin? What selective pressures encouraged the evolution of light skin? One positive effect of UV radiation is that it synthesizes vitamin D for us, which is an essential vitamin. We can get vitamin D from foods, but the main source is from the sun. Vitamin D is key for mineralizing bones and normal growth. A lack of vitamin D can lead to rickets which produces soft bones and pelvic deformities. As people moved from the tropics to more temperate areas, reduced sunlight and the need to wear more clothes would select for lighter skin to regulate Vitamin D synthesis.

ricketsx-ray of rickets

Children with bowed legs resulting from rickets; X-ray of the condition.

Here are two articles on the evolution of human skin color:




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